Pre-manufactured door frames are used extensively in construction because they can be rapidly manufactured in large volumes to consistently high quality standards and close tolerances, thus significantly reducing the time and cost of job-site labour that would be required if a craftsman had to manufacture a custom door frame. Pre-manufactured door frames can be quickly and easily installed in a wood framed door aperture or in a masonry door opening by relatively unskilled persons with simple tools to yield a finished door frame of professional quality and appearance.
Because the walls in which wood framed door apertures or masonry door openings are situate can be of varying thicknesses, pre-manufactured door frames are sometimes constructed so that they may be adjusted to fit a range of wall thicknesses. Conventionally, premanufactured adjustable door frames have six major parts; namely, a two part strike jamb assembly, a two part hinge jamb assembly, and a two part header assembly. One part of each pair is ordinarily provided with a slotted groove or channel member which slidably receives a mating tongue or channel member of the other part. To assemble the door frame a workman fits the three grooved parts into position on one side of the door aperture and fastens them in place. The other three parts are then slid into position from the opposite side of the door aperture so that the aforementioned tongues or channel members are slidably received within the cooperating grooves or channel members in the first parts. The tongued (or channelled) parts are slidably positioned as aforesaid until their outer flanges are flush with the wall and the tongued (or channelled) parts are then also fastened to the wall to complete the door frame.
Tongue and groove type expandable door frames suffer at least two disadvantages. First, they can only be adjusted to accomodate a relatively small variation in wall thickness (typically about 1/2 inch). Accordingly, manufacturers of such door frames have had to provide a range of six or more models merely to accomodate wall thicknesses varying from about four to seven inches. This requires the manufacturer to schedule production volumes carefully, maintain increased product inventory and check customer orders carefully. The customer must in turn carefully determine the size of door frame required to fit each of the door apertures in a particular building and ensure that the correct door frame model is ordered for each aperture.
Expandable door frames constructed with channel members are capable of accomodating somewhat greater wall thickness variation than tongue and groove type expandable door frames, but it can be difficult and time consuming to align the channel members so that the door frame parts can be slid into position.
The second disadvantage of tonque and groove type expandable door frames is that the tongue and groove interlock is relatively flimsy and can in some cases cause misalignment of the door frame.
The present invention provides a pre-manufactured expandable door frame which overcomes the foregoing disadvantages. More particularly, expandable door frames constructed in accordance with the invention are of extremely rigid construction and may be adjusted to accomodate wall thickness variations of about two and one quarter inches or more, thus affording manufacturers the significant advantage of focusing their manufacturing, inventory and ordering on a single door frame model adapted to suit the widest possible range of customer needs. A further significant advantage of the preferred embodiment is that it may be prehung with a door at the factory.